1. Go to platformui admin interface and to "Content -> Content Structure";

2. Choose to create a content specifically of type article, which has a required richtext fieldtype;

3. On its creation form, click on the richtext field "intro" (the required one) and choose to embed a content;

4. On the UDW, pick some content and confirm. You will be taken back to the article creation form.
The "intro" field despite being filled in with an embedded content, will still notice the user that the field is mandatory to be filled in (directly putting input text onto it however validates it as filled in).